1. Field of the Invention
This invention relates to an active noise attenuating device provided in a propagation path of noise for producing a sound having the same amplitude as that of the noise and a phase opposite to the noise, to cause a sound interference between the noise and the produced sound, thereby attenuating the noise.
2. Description of the Prior Art
An active noise attenuating device has recently been proposed for attenuating noise produced by an air conditioner and propagating along a draft duct thereof. The active noise attenuating device produces a sound having the same amplitude as that of the noise and a phase opposite to the noise to cause a sound interference in the draft duct, thereby actively attenuating the noise and reducing an amount of noise leaking out of the draft duct.
An active noise attenuating technique applied to the above-described device employs applied electronic techniques and particularly, an acoustic data processing circuit arrangement and acoustic interference. In this active noise attenuating technique, basically, a sound receiver such as a microphone is provided in the draft duct to detect the sound from a noise source, thereby converting the detected sound to a corresponding electrical signal. The electrical signal is processed into a signal by an operation unit. The signal is supplied to a control sound producer such as a loud speaker so that it produces an artificial sound having the same amplitude as of the noise and the phase opposite to the noise, at a control point and so that the artificial sound interferes with the noise at the control point. Consequently, an attenuation efficiency can be expected to amount to 10 dB or more in a low frequency band in the above-described device. Moreover, no pressure loss occurs in the above noise attenuating device. For example, when a concert hall is equipped with the above-described active noise attenuating device, noise produced from the draft ducts can be attenuated such that a better space can be provided for appreciation of music.
In employment of the active noise control in practice, characteristic variations due to aged deterioration of parts composing the signal system and due to an ambient temperature need to be coped with. For this purpose, an operational factor or acoustic transfer function of the operation unit is adjusted in accordance with variations in the noise attenuating performance of the device. More specifically, a monitoring sound receiver such as a microphone is provided for monitoring the noise attenuating effect of the control sound producer. Adaptive control means is also provided for controlling the operation unit. When the monitorial result is out of a predetermined allowable range, the adaptive control means changes the operational factor of the operation unit so that the monitorial result is within the allowable range. Consequently, the noise attenuation performance in the active noise control is maintained at its optimum in accordance with the characteristic variations. This control manner is referred to as "adaptive control."
FIGS. 7 and 8 illustrate an example of the conventional active noise attenuating device as described above. Referring to FIG. 7, a duct 1 has a closed end and an open end. A noise source 2 is disposed at the side of the closed end in the duct 1. An active noise attenuating device 3 is provided for preventing noise produced from the noise source 2 from leaking out of an opening la of the duct 1. A sound source microphone 4 for detecting noise, a loud speaker 5 producing an interference sound and a monitoring microphone 6 as will be described later are disposed at respective points S, A and O along a noise propagation path in the duct 1. The noise from the noise source 2 is detected by the microphone 4 at point S, and the microphone 4 generates a detection signal indicative of the detected noise. The detection signal is supplied to a control section 7 in which the detection signal is processed into a control sound such that the sound pressure becomes zero by acoustic interference at point O in the vicinity of the opening 1a when it is produced from the loud speaker 5. When the control sound is produced from the loud speaker 5 to be directed to the opening la of the duct 1, the acoustic interference is caused between the noise and the control sound such that a so-called acoustic wall is formed. The noise is prevented by the acoustic wall from leaking out of the opening 1a. The monitoring microphone 6 measures an amount of attenuated noise at point O and generates a detection signal indicative of the measured amount of attenuated noise. The detection signal is supplied to the control section 7. The control section 7 previously measures the acoustic transfer characteristic of the duct 1 and the transfer characteristics of the sound source microphone 4 and the loud speaker 5 in order that a signal for producing the control sound is generated. Based on the results of the measurement, the characteristic of a filter for processing the detection signal from the sound source microphone 4 is obtained.
A method of obtaining the filter characteristic will be described. In the following description, reference symbol G.sub.AO designates the acoustic transfer characteristic of a transfer path between point A indicative of the location of the loud speaker 5 and point O indicative of the location of the monitoring microphone 6. Reference symbol G.sub.SO designates the acoustic transfer characteristic of a transfer path between point S indicative of the location of the sound source microphone 4 and point O. Reference symbol G.sub.SA designates the acoustic transfer characteristic of a section between point S where the noise is received by the microphone 4 and point A where the control sound obtained by processing the detection signal indicative of the received sound is produced. First, a random noise such as a white noise is produced from the loud speaker 5 so that the acoustic transfer characteristic G.sub.AO is measured. The acoustic transfer characteristic G.sub.SO is then measured in the condition that the random noise is being produced from the loud speaker 5. Then, the acoustic transfer characteristic G.sub.SO can be shown by the following expression: EQU G.sub.SO =G.sub.SA .multidot.G.sub.AO. (1)
Consequently, the transfer characteristic G of a filter of the control section 7 needs to have an opposite phase with the acoustic transfer characteristic G.sub.SA. From the equation (1), the transfer characteristic G of the filter is obtained as follows: EQU G=-G.sub.SA =-G.sub.SO /G.sub.AO. (2)
Accordingly, the noise can be attenuated by the control sound produced from the loud speaker 5 at point O indicative of the location of the monitoring microphone 6 when the transfer characteristic G of the filter is set for a value shown by the equation (2).
In order that a sufficient noise attenuating effect is always achieved, the control sound needs to be automatically adjusted in consideration of the variations of the acoustic transfer characteristic in the duct 1 due to aged deterioration of the sound source microphone 4 and the loud speaker 5 and the changes in the ambient temperature and the like. For this purpose, the prior art has proposed active noise attenuation system of the adaptive control type as disclosed in Japanese Unexamined Patent Application Publication No. 61-296392. In the disclosed system, sound unattenuated by the control sound is detected by a monitoring microphone disposed at point O or an aural null. A control section is controlled in a feedback manner so that an amount of the sound detected by the monitoring microphone is rendered the minimum, whereby the noise attenuation effect is maintained at a high level.
FIG. 8 illustrates the arrangement of the control section in the noise attenuation system of the adaptive control type as described above. The detection signal from the sound source microphone 4 is supplied both to a noise attenuating filter 8 and to another filter 9 which is set for the transfer characteristic G.sub.AO of the transfer path between the loud speaker 5 and the monitoring microphone 6. An adaptive filter 10 is supplied with a signal from the filter 9 and a detection signal from the monitoring microphone 6 via an operation unit 11. In this system, too, the acoustic transfer characteristic G.sub.AO of the transfer path between the loud speaker 5 and the monitoring microphone 6 is previously obtained in the same manner as described above. A detection signal y indicative of the sound detected at point O is shown by the following expression: EQU y=G.sub.SO .multidot.x (3)
where x is a detection signal indicative of the noise reaching the sound source microphone 4. A signal -y having a phase opposite to the detection signal y needs to be superimposed on the signal y at point O in order that the signal y is rendered zero. The signal -y is obtained from the following equation: EQU -y=G.sub.AO .multidot.a (4)
where a is a signal indicative of the sound produced from the loud speaker 5. Using G for the characteristic of the noise attenuating filter 8, EQU a=G.multidot.x=-G.sub.SO /G.sub.AO .multidot.x (5)
and EQU y=(-G).multidot.G.sub.AO .multidot.x. (6)
Accordingly, -G is obtained from the detection signal y of the monitoring microphone 6 and the signal G.sub.AO .multidot.x obtained by processing the detection signal x of the sound source microphone 4 by the filter 9 with the acoustic transfer characteristic G.sub.AO, by way of identification by the adaptive filter 10 and the operation unit 11. Then, the characteristic of the filter 8 is obtained by way of sign change. When a digital filter is used for the processing, the characteristic is obtained in the form of a filter factor. Accordingly, the sign change can be obtained by subtracting each tap factor value from zero.
When the spatial acoustic transfer characteristic G.sub.SO is shifted to G.sub.SO ' by environmental changes or the like, the optimum value G.sub.new of the characteristic of the noise attenuating filter 8 is shifted by .DELTA.G relative to the present noise attenuating filter characteristic G.sub.old. The optimum value G.sub.new is shown by the following expression: EQU G.sub.new =G.sub.old -.DELTA.G. (7)
In this case, the detection signal y' indicative of the unattenuated noise detected at point O is shown by the following equation: EQU y'=x.multidot.G.multidot.G.sub.AO +x.multidot.G.sub.SO '. (8)
Accordingly, the relationship at the time of an optimum noise attenuation is shown by the following equation: EQU x(G-.DELTA.G).multidot.G.sub.AO +x.multidot.G.sub.SO '=0. (9)
Eliminating G.sub.SO ' from equations (8) and (9), we have: EQU y'=x.multidot.G.multidot.G.sub.AO -x.multidot.(G-.DELTA.G).multidot.G.sub.AO =(x.multidot.G.sub.AO).multidot..DELTA.G. (10)
Accordingly, the deviational component .DELTA.G of the filter characteristic G is obtained from the detection signal y' from the monitoring microphone 6 and the signal G.sub.AO .multidot.x obtained by processing the sound source signal x by the filter 9 having the filter characteristic G.sub.AO, by way of identification by the adaptive filter 10 in the same manner as in the equation (6). Consequently, a new optimum noise attenuation filter characteristic of the filter 8 can be obtained from the equation (7). When the equation (6) is compared with those (7) and (10), the acoustic transfer characteristic G of the noise attenuation filter 8 to be initially obtained corresponds to the value of the equation (7) where 0 is substituted for G.sub.old. Thus, an optimum noise attenuation can be obtained when the adaptive processing and the factor renewal processing represented respectively by the equations (10) and (7) where 0 is substituted for the initial value of the characteristic of the filter 8 are repeated. Actually, however, the factor renewal is performed by multiplying .DELTA.G by a feedback gain parameter .mu. rather than using the equation (7) as follows: EQU G.sub.new =G.sub.old -.mu..DELTA.G. (11)
In the case where the feedback gain parameter .mu. is employed, the convergence speed and the stability can be improved or adjusted advantageously. However, the acoustic transfer characteristic G.sub.SO of the transfer path between the loud speaker 5 and the sound source microphone 4 and the acoustic transfer characteristic G.sub.AO of the transfer path between the loud speaker 5 and the monitoring microphone 6 need to be measured and identified prior to initiation of the noise attenuating operation. Accordingly, when the noise attenuating control is started in the condition that the noise is being produced from the noise source 2, an accurate identification cannot be performed, which provides insufficient noise attenuating effect.
In view of the above-described problem, the active noise attenuating device is conventionally started first and the identification of the acoustic transfer characteristics is then performed. Thereafter the air conditioner or the like which is a noise source 2 is started. Accordingly, since the air conditioner cannot be driven at once when connected to a power source, the air conditioning operation cannot be performed promptly. Furthermore, the acoustic transfer characteristic G.sub.AO varies out of the range of the adaptive control by the adaptive filter 10 when the changes in the temperature, the aged deterioration or the like changes the condition of the duct 1 during the noise attenuating operation. Since the noise attenuating effect by the active noise control is lowered in such a case, the acoustic transfer characteristic need to be reidentified. However, the acoustic transfer characteristic can be identified only when the noise is not produced from the noise source 2. Accordingly, the air conditioner needs to be once turned off for the purpose of execution of the identification of the acoustic transfer characteristic. Consequently, the efficiency in operation of the air conditioner is lowered.
In view of the foregoing, the following countermeasure has been proposed. An identifying sound louder than the noise produced from the noise source 2 is produced from the loud speaker 5 so that the signal-to-noise (S/N) ratio of the identifying sound relative to the noise is increased, whereby the acoustic transfer characteristic can be identified even during drive of the air conditioner. However, the sound louder than the noise is produced from the loud speaker 5 provided for attenuating the noise, during the processing of the identification of the acoustic transfer characteristic. Thus, the active noise attenuating device cannot perform its function during this processing and moreover, the device itself becomes a noise source. Consequently, the above-described countermeasure is impractical.